All you need to do is open the file Info.plist, which resides in <Install Folder>/jEdit/jEdit.app/Contents (right click and use "Show Package Contents" on the .app if you're doing this through the GUI), and add a new key to the Java entry (assuming it doesn't already exist). The new key should be called Properties, and its value should be a dict. The Properties dictionary (whether already there, or newly created by you) needs to have a key called com.apple.macos.useScreenMenuBar with a string value that is true.

This is most easily accomplised using the Property List Editor, which pops up when you double-click on a .plist. But it can also be done using a plain old test editor (hooray for xml!). Either way, the end result should be the same: Info.plist should look have an entry inside the Java dictionary that looks like the example below

<plist version="1.0">
<dict>
   ...
        <key>Java</key>
        <dict>
              ...
                <key>Properties</key>
                <dict>
                        <key>com.apple.macos.useScreenMenuBar</key>
                        <string>true</string>
                </dict>
              ...
        </dict>
   ...
</dict>

Hope that helps...

Allan